Already known from Patent Application FR No. 2 177 581 is a means of separating waters polluted by hydrocarbons comprising a buffer tank acting as a pre-separator and comprising a central tube mounted telescopically and provided at its end with a funnel connected to floats. This funnel is intended to recover the floating hydrocarbons. However, the position of the skimming float, which causes the skimming to vary, depends upon the flow values for throughput of treated water which has to be regulated and maintained accordingly. This disadvantage is unacceptable in the circumstances occurring in production fields, because of the fluctuations in the rate of flow of polluted waters. Furthermore, this document does not employ the induced air floatation technique.
It is likewise known to continuously treat waters polluted with hydrocarbons or other substances which are not miscible with water, in floatation apparatus employing induced air floatation. However, such equipment is not adapted to the petroleum industry and has particular drawbacks where security and safety are concerned, as well as in operation and processing.
Indeed, in petroleum fields, induced air floatation equipment operates under a gas covering so as to avoid contact with oxygen for safety reasons. The reservoirs or capacities are therefore subject to hydrostatic pressure due to the liquid and the pressure of the gas covering which is around 20 to 30 mbars. The floatation equipment according to the prior art is of parallelepiped shape and is constructed to withstand pressures of around 100 to 200 mbars. In oil fields, having regard to fluctuations in water production, and having regard to the lack of viability of certain control elements (water/oil interface detector), pressures of plural bars may be observed at the input to the floatation equipment. These pressures can give rise to considerable leakage of liquid and gases or may even result in complete destruction of the equipment. When it is known that the petroleum gases may contain toxic products (for example H2S), it will be appreciated that such a risk of leakage is unacceptable.
At operating level, induced air floatation equipment requires fine regulation of the internal liquid level since this level determines the skimming rate of the foam involved. Having regard to fluctuations in flow in oil fields, it is difficult to maintain the internal level within an acceptable range of variation and furthermore to check that the adjustment has been correctly made.
Finally, having regard to the difficulties in regulation set out hereinabove, it is necessary at processing level to have a considerable rate of skimming, generally more than 10% of the flow through the equipment. The skimmed liquid carrying an oily foam and containing a floatation adjunct (a polyelectrolyte) has to be recycled, which on the one hand reduces the nominal rate of flow of the processing chain while on the other it opposes the primary separation of oils by the presence of floatation additives in gravity separators.